Control Diagnostic Apparatus for Internal Combustion Engine

ABSTRACT

To prevent the deterioration of exhaust emissions of an internal combustion engine due to air-fuel ratio variation among multiple cylinders, and to identify an abnormal cylinder during an abnormality of cylinder air-fuel ratio variation. An control apparatus of an internal combustion engine comprising upstream air-fuel ratio detection means for detecting upstream air-fuel ratio of the catalyst that purifies exhaust emissions discharged from multiple cylinders, and configured to control air-fuel ratio of the multiple cylinders based on the upstream air-fuel ratio, wherein the air-fuel ratio variation among the multiple cylinders is increased and the upstream air-fuel ratio is controlled to become rich. Further, an abnormality of the air-fuel ratio variation among the multiple cylinders and an abnormal cylinder are identified based on an output of an air-fuel ratio sensor in downstream of the catalyst when the air-fuel ratio variation is increased, or an estimated value of an air-fuel ratio (central air-fuel ratio) at which the purification efficiency of the catalyst becomes optimal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a control apparatus for an internalcombustion engine having multiple cylinders.

2. Background Art

In an internal combustion engine having an exhaust gas purificationsystem that utilizes a three-way catalyst, in order to perform thepurification of HC, CO, and NOx in the exhaust gas with a catalyst,control of the air-fuel ratio of the gaseous mixture to be combusted inthe internal engine is performed to achieve an air-fuel ratio (a centralair-fuel ratio) at which the three components: HC, CO, and NOx arepurified at a high efficiency in a good balance. Such control ofair-fuel ratio is realized by providing an air-fuel ratio sensor in anexhaust passage of the internal combustion engine and performing afeedback control to make an air-fuel ratio detected by the sensorcorrespond to a predetermined target air-fuel ratio. (See JP PatentPublication (Kokai) No. 2009-30455.)

SUMMARY OF THE INVENTION

However, in an internal combustion engine having multiple cylinders,there is a drawback that the exhaust emission performance of theinternal combustion engine may be deteriorated due to variation of theair-fuel ratio among the multiple cylinders (the air-fuel ratio of eachcylinder).

It is an object of the present invention to prevent the deterioration ofthe exhaust emission performance of an internal combustion engine due toair-fuel ratio variation among the multiple cylinders.

A control apparatus of an internal combustion engine comprises: acatalyst for purifying exhaust gas discharged from multiple cylinders;upstream air-fuel ratio detection means for detecting an upstreamair-fuel ratio of exhaust gas that flows into the catalyst; and air-fuelratio control means for controlling a fuel injection amount of themultiple cylinders based on the upstream air-fuel ratio, wherein thecontrol apparatus is adapted to control the fuel injection amount of themultiple cylinders when the air-fuel ratio among the multiple cylindersvaries, such that the upstream air-fuel ratio is richer than theupstream air-fuel ratio detected before the air-fuel ratio among themultiple cylinders varies.

Further, upon application of means relating to the present invention, itis possible to identify an abnormality of air-fuel ratio variation amongthe multiple cylinders and an abnormal cylinder based on an output of anair-fuel ratio sensor detected in a downstream of the catalyst when theair-fuel ratio variation is actively increased, or an air-fuel ratio(central air-fuel ratio) at which three exhaust emission components (HC,CO, and NOx) react with oxygen in the exhaust gas without excess ordeficiency.

According to the present invention, it is possible to prevent thedeterioration of the exhaust emission performance of an internalcombustion engine due to air-fuel ratio variation among the multiplecylinders.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general configuration diagram of an internal combustionengine.

FIG. 2 is an explanatory diagram to illustrate the principle of air-fuelratio variation among the multiple cylinders.

FIG. 3 illustrates the effects of air-fuel ratio variation among themultiple cylinders (air-fuel ratio sensor output and exhaust emissions).

FIG. 4 illustrates the effects of air-fuel ratio variation among themultiple cylinders (rear oxygen sensor output and exhaust emissions).

FIG. 5 is an exemplary control block diagram of the present invention.

FIG. 6 illustrates an exemplary target air-fuel ratio computationsection.

FIG. 7 illustrates an exemplary method of correcting the target air-fuelratio and the central air-fuel ratio.

FIG. 8 illustrates the relationship between the degree of air-fuel ratiovariation and an optimum purification air-fuel ratio.

FIG. 9 is a time chart when the present invention is implemented (duringnormal time).

FIG. 10 is a time chart when the present invention is implemented(during a rich abnormality of the No. 1 cylinder).

FIG. 11 illustrates a change amount of the central air-fuel ratio duringnormal time.

FIG. 12 is a time chart during normal time.

FIG. 13 illustrates a change amount of the central air-fuel ratio duringa rich abnormality of the No. 1 cylinder.

FIG. 14 is a time chart during a rich abnormality of the No. 1 cylinder.

FIG. 15 illustrates a change amount of the central air-fuel ratio duringa lean abnormality of the No. 1 cylinder.

FIG. 16 is a time chart during a lean abnormality of the No. 1 cylinder.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereafter, embodiments of the present invention will be described withreference to the drawings.

FIG. 1 is an exemplary general configuration diagram of an internalcombustion engine to which the present invention is applied. At ECU 108(control apparatus), control is performed such that the amount of airthat flows into each cylinder is adjusted with a throttle 104, and afuel injection amount of an injector 105 is adjusted such that anair-fuel ratio of each cylinder in the engine head 106 becomes apredetermined value (a target air-fuel ratio). In order to determine thefuel amount of the injector 105, a base injection amount which serves asa basics is determined by estimating the air amount in the cylindersfrom an air amount detected by an airflow rate sensor 103 and arevolutional speed detected by a revolutional speed sensor (not shown).The base injection amount is subject to a fuel correction such that anoutput (an upstream air-fuel ratio) of an air-fuel ratio sensor 101placed in the upstream of a catalyst 107 corresponds to a targetair-fuel ratio to be targeted. Further, a rear oxygen sensor 102detecting a down stream air-fuel ratio in the downstream of the catalyst107 is used to calculate (compute) an air-fuel ratio (a central air-fuelratio) at which three exhaust components (HC, CO, NOx) fully react withoxygen without excess or deficiency. Then, controlling the targetair-fuel ratio to be near the central air-fuel ratio allows the exhaustemission performance of the internal combustion engine to be maintainedhigh even if there is a certain level of errors in the estimates ofairflow amount and the output of the air-fuel ratio sensor. However, iffor example the injector 105 is degraded resulting in variation of theinjection amount of each cylinder, and an abnormality that the air-fuelratio of each cylinder varies (air-fuel ratio variation among themultiple cylinders) occurs, deterioration of exhaust emissions willresult through a mechanism to be described next.

FIG. 2 schematically shows the relationship between the output of theair-fuel ratio sensor in the upstream of the catalyst (measured values)and the actual air-fuel ratio of exhaust gas (true values) when theair-fuel ratio of a predetermined cylinder is intentionally shifted tothe rich side or lean side thereby generating an air-fuel ratiovariation among the multiple cylinders. Even if the air-fuel ratio amongthe multiple cylinders is varied, the measured value detected by theair-fuel ratio sensor 101 is controlled to become constant at a targetvalue. This is because as described above, a feedback control serves tomaintain measured values at the target value. However, as shown in FIG.2, the air-fuel ratio of the exhaust gas that actually flows into thecatalyst becomes leaner as the air-fuel ratio of a predeterminedcylinder is shifted away from the stoichiometric value, and the air-fuelratio variation among the multiple cylinders increases. In other words,a measured value will become richer than the true value due to theair-fuel ratio variation among the multiple cylinders. It is noted thatthe air-fuel ratio of exhaust gas described herein is an air-fuel ratiothat can be calculated by a common method from the concentrations ofexhaust components (HC, CO, CO₂) in the upstream of the catalyst and soon, and is calculated, for example, as the output of an exhaust gasanalyzer.

FIG. 3 illustrates an example of experimental results that confirmed a“rich shift” in the air-fuel ratio sensor 101 output caused by anair-fuel ratio variation among the multiple cylinders. FIG. 3( a) showsoutput values of the air-fuel ratio sensor in the upstream of thecatalyst in the abscissa axis, and measured values of HC, CO, and NOx inthe downstream of the catalyst measured by an exhaust gas analyzer inthe ordinate axis. If there is no air-fuel ratio variation among themultiple cylinders, the central air-fuel ratio of HC, CO, and NOx is14.45. However, if the air-fuel ratio of the No. 1 cylinder is madericher by 20% than those of the other three cylinders, the centralair-fuel ratio is shifted by 0.1 toward the rich side to be 14.35. Thatis, this experimental result shows that when there is an air-fuel ratiovariation among the multiple cylinders, the exhaust gas air-fuel ratiobecomes lean even if the air-fuel ratio sensor 101 output is maintainedto be constant by air-fuel ratio control, thereby resulting in anincrease in the NOx exhaust amount.

The reason why the measured value become richer than the true value isthat the rich response of the air-fuel ratio sensor 101 is faster thanthe lean response thereof. Therefore, this phenomenon becomes noticeableas the difference between the rich response and the lean response of thesensor increases. On the other hand, this difference is small in therear oxygen sensor 102 placed in the downstream of the catalyst, and arich shift as described above will not occur.

FIG. 4 shows an example of the relationship between the rear oxygensensor output and the exhaust gas in the downstream of the catalyst. Inthe present experiment, the air-fuel ratio of the No. 1 cylinder is setto be the same (normal) as, 20% richer, and 20% leaner than that ofother cylinders. In the present experiment, the sensor output when HC,CO, NOx are best purified is about 600 mV regardless of the air-fuelratio variation among the multiple cylinders. Therefore, setting atarget air-fuel ratio for the air-fuel ratio control such that theoutput value of the rear oxygen sensor (the rear O₂ sensor) becomesconstantly about 600 mV will allow the prevention of exhaust emissiondeterioration due to air-fuel ratio variation among the multiplecylinders.

Embodiment 1

A first embodiment of the present invention will be described by usingFIGS. 5 to 10.

FIG. 5 is an exemplary control block diagram to implement the presentinvention. As will be described later in detail, a target air-fuel ratiocomputation section 501 calculates an oxygen storage amount (OS amount)in the catalyst from an air-fuel ratio sensor output (the upstreamair-fuel ratio) and an airflow rate sensor output, and computes a targetair-fuel ratio which causes the OS amount to stay in a predeterminedrange. A base injection amount computation section 503 estimates acylinder air amount from an airflow rate sensor output and an enginerevolutional speed sensor (not shown) output, etc. and calculates a baseinjection amount based on the target air-fuel ratio. An air-fuel ratiocontrol computation section 504 computes a correction value (an air-fuelratio correction amount) for the base injection amount which will makethe air-fuel ratio sensor output correspond to the target air-fuelratio. Then, in an INJ injection amount setting section 506, a fuelpulse width which is obtained by applying the air-fuel ratio correctionamount to the base injection amount is set for each cylinder. Thepresent embodiment includes a cylinder variation control computationsection 505 that computes a command value for variation of the upstreamair-fuel ratio among the multiple cylinders (for cylinder air-fuel ratiovariation) to realize a cylinder variation of each cylinder, and acylinder air-fuel ratio variation determination section 502 thatdetermines the cylinder air-fuel ratio variation from the command valuefor variation of the air-fuel ratio among the multiple cylinders and theoutput of the air-fuel ratio sensor in the downstream of the catalyst.Correcting the target air-fuel ratio by the control of the fuelinjection amount to the rich side depending on the cylinder air-fuelratio variation set value which has been set here allows the preventionof the deterioration of exhaust emissions due to air-fuel ratiovariation among the multiple cylinders. Further, it is possible todetermine an abnormality of cylinder air-fuel ratio variation based onthe rear oxygen sensor output (the down stream air-fuel ratio) while anair-fuel ratio variation among the multiple cylinders is generated.

FIG. 6 illustrates an example of the target air-fuel ratio computationsection. An oxygen storage amount calculation section 602 calculates anoxygen storage amount OS by the following equation 1 from an air-fuelsensor output RABF an airflow rate sensor output QA and a centralair-fuel ratio CNTABF.

OS=Σ(RABF−CNTABF)*QA  Equation 1

A target air-fuel ratio correction section 601 changes the targetair-fuel ratio in the direction in which the oxygen storage amount andthe rear oxygen sensor output return to predetermined ranges when theoxygen storage amount OS departs from the predetermined range or theoutput of the rear oxygen sensor placed behind the catalyst departs fromthe predetermined range. Further, a central air-fuel ratio estimationsection 603 corrects the central air-fuel ratio when the oxygen storageamount is within the range and the rear oxygen sensor output departsfrom the predetermined range. An air-fuel ratio correction section 604corrects the target air-fuel ratio and the central air-fuel ratio basedon the cylinder air-fuel ratio variation.

FIG. 7 illustrates an example of the method of correcting the targetair-fuel ratio and the central air-fuel ratio. For example, when theoxygen storage amount OS increases, the target air-fuel ratio is madericher. Since this will cause the air-fuel ratio of the exhaust gas thatflows into the catalyst to become richer thereby reducing the oxygenstorage amount, it is possible to prevent the decline of NOxpurification rate before a decline of NOx purification rate is detectedby the rear oxygen sensor. Moreover, when the rear oxygen sensor outputexceeds a rich determination criterion even though the oxygen storageamount is increasing, the target air-fuel ratio is made richer and thecentral air-fuel ratio is corrected to become richer to modify theoxygen storage amount computation of Equation 1. This will allow theair-fuel ratio of the exhaust gas that flows into the catalyst toapproach the central air fuel ratio at which three exhaust componentsreact with oxygen without excess or deficiency in the presence of thecatalyst.

FIG. 8 illustrates the relationship between the degree of variation ofair-fuel ratio and an optimum purification air-fuel ratio. In thepresent example, air-fuel ratio variation is realized by increasing thefuel pulse width in one predetermined cylinder. Moreover, the optimumpurification air-fuel ratio represents an output value of the air-fuelratio sensor in the upstream of the catalyst, which is detected when allof HC, CO, NOx are purified at a highest efficiency by the catalyst. Inthe present invention, the injection pulse width of a predeterminedcylinder is increased and the target value of the air-fuel ratio controlis shifted to the rich side. In particular, when the control apparatusgraphically makes the air-fuel ratio variation among the multiplecylinders, the response of the control of the air-fuel ratio becomesfaster by setting a rich shift amount from an optimum air-fuel ratiodetermined by experiment in advance, than by correcting the targetair-fuel ratio to an optimum air-fuel ratio based on the rear oxygensensor output. As a result, the above setting a rich shift prevents fromthe deterioration of exhaust emissions.

FIG. 9 is an example of a time chart when the present control isperformed. The control apparatus starts to make an air-fuel ratiovariation among the multiple cylinders by increasing the fuel pulsewidth of the No. 1 cylinder to be larger than those of other cylinders,and terminates to make the variation among the multiple cylinders bystopping the above increase. In the present embodiment, the controlapparatus starts to make an air-fuel ratio variation among the multiplecylinders and simultaneously controls the upstream air-fuel ratio (theoutput of the air-fuel ratio sensor) by sifting the target air-fuelratio toward the rich side.

As a result, an actual air-fuel ratio at the entrance of the catalyst ismaintained at an air-fuel ratio at which the purification efficiency ofthe catalyst is optimized, and the rear oxygen sensor output ismaintained between a rich determination threshold value and a leandetermination threshold value. On the other hand, when terminating theair-fuel ratio variation among the multiple cylinders, the targetair-fuel ratio controlled (shifted) conversely toward the lean side.

FIG. 10 is an example of the time chart when the present control isperformed during the making of an air-fuel ratio variation among themultiple cylinders. In this case, the rear oxygen sensor output is lowerthan the lean determination value even though the target air-fuel ratiois controlled (shifted) to become rich. This indicates that the air-fuelratio of the No. 1 cylinder is richer than those of other cylindersbefore the fuel injection amount (the injection pulse width) of the No.1 cylinder is increased. In this case, the injection pulse widths ofother cylinders are increased one cylinder by one cylinder to recordwhether the rear oxygen sensor output departs from the range toward therich side or the lean side. If the rear oxygen sensor output becomeslean when the fuel injection is increased in the No. 1 cylinder, andbecomes rich when the fuel injection is increased in other cylinders, itcan be determined that the No. 1 cylinder is in a rich abnormality. Thereason is that if a rich cylinder is made richer, the air-fuel ratiovariation among the multiple cylinders increases more than expected, andthe rich correction predetermined by experiment is insufficient causingthe rear oxygen sensor output to be lean. On the contrary, if anothercylinder which has been made lean to balance with a rich cylinder ismade richer, the rear oxygen sensor output becomes rich since theair-fuel ratio variation among the multiple cylinders becomes smallerthan expected and the rich correction is large. When the No. 1 cylinderis in a lean abnormality, vice versa is true and if the control ofair-fuel ratio variation among the multiple cylinders is performed forall the cylinders and the rear oxygen sensor output becomes rich only inone predetermined cylinder, the one cylinder can be determined to be ina lean abnormality. Further, inhibiting the determination when theoxygen storage amount OS departs from a predetermined range allows theprevention of erroneous determination due to external disturbances. Thepresent control is preferably performed after the catalyst issufficiently activated.

Implementing the above described embodiment will achieve the followingadvantages. It is possible to prevent the deterioration of exhaustemissions when an air-fuel ratio variation among the multiple cylindersis intentionally generated. Therefore, the present control can improvethe purification efficiency of the catalyst and facilitate theactivation of the catalyst by making the air-fuel ratio chatter aroundan optimum purification air-fuel ratio, thereby improving exhaustemission performance. Further, it is possible to detect an abnormalityof air-fuel ratio variation among the multiple cylinders and to identifyan abnormal cylinder of which air-fuel ratio is different from those ofothers based on the rear oxygen sensor output upon generating theair-fuel ratio variation among the multiple cylinders.

Embodiment 2

A second embodiment will be described by using FIGS. 11 to 16. In thesecond embodiment, a central air-fuel ratio is used to performabnormality determination. In the present embodiment, as shown in FIG.11( a), when a particular cylinder is controlled to become richer by aproportion X, the target air-fuel ratio and the central air-fuel ratioare shifted to the rich side by Y0. The relation between X and Y0 isdetermined by the above described optimum purification air-fuel ratio.FIG. 11( b) shows a change amount of the central air-fuel ratio beforeand after the diagnostics. During normal time, air-fuel ratio among themultiple cylinders becomes uniform. Since the central air-fuel ratio iscorrected from A to B of FIG. 11( a), the change amount of the centralair-fuel ratio will be Y0 when whichever cylinder is made rich.

FIG. 12 illustrates a time chart during normal time. In this embodiment,only the No. 1 cylinder is controlled to become richer by a richproportion X upon start of diagnostics, and the target air-fuel ratioand the central air-fuel ratio are made richer by Y0 depending on therich proportion X. As described in FIG. 11( a), X and Y0 shown here area rich proportion and a rich shift amount of air-fuel ratio formaintaining an optimum purification air-fuel ratio. As a result of this,the catalyst will not depart from the optimum purification air-fuelratio, and the output voltage of the rear oxygen sensor for detectingthe air-fuel ratio in the downstream of the catalyst will be maintainedwithin a predetermined range (600 to 800 mV).

Next, description will be made on a case in which one cylinder is richand an air-fuel ratio variation among the multiple cylinders hasoccurred. FIG. 13( a) illustrates an optimum air-fuel ratio when controlof the air-fuel ratio variation among the multiple cylinders isperformed, in an abnormality case in which one cylinder is rich. Arrepresents an air-fuel ratio before performing cylinder air-fuel ratiovariation control and the No. 1 cylinder is shifted toward the rich sideby Xre. Then, in order to keep the air-fuel ratio at a target air-fuelratio, other cylinders are shifted toward the lean side by Xre/(n−1).Where, n represents the number of cylinders. The result of shifting theNo. 1 cylinder which is in a rich abnormality toward the rich side by Xis Br, and the result of shifting the normal cylinders excepting the No.1 cylinder toward the rich side by X is Bl. Thus, a cylinder in which arich abnormality has occurred becomes richer than an expected value ofY0. On the other hand, when other normal cylinders are made rich, theywill become leaner than an expected value of Y0. FIG. 13( b) illustratesthe record of the central air-fuel ratio change amount when eachcylinder is shifted toward the rich side by X, in which only thecylinder of a rich abnormality changes its central air-fuel ratio towardthe rich side by a significant amount. Thus, if the recorded value ofthe central air-fuel ratio change amount becomes as shown in FIG. 13(b), it can be determined to be an abnormality of central air-fuel ratio,and one cylinder can be determined to be in a rich abnormality becauseonly it is richer than Y0.

FIG. 14 is an example of a time chart when the No. 1 cylinder is in arich abnormality. Only the No. 1 cylinder is shifted toward the richside by X, and the target air-fuel ratio and the central air-fuel ratioare controlled to become richer. However, since as shown in FIG. 13( a),the optimum purification air-fuel ratio at that time is further richer,NOx cannot be purified and at the same time the output of the rearoxygen sensor in the downstream of the catalyst will become lean.Moreover, since at this time, the oxygen storage amount is not out of apredetermined range, the target air-fuel ratio and the central air-fuelratio are corrected toward the rich side. As a result of thiscorrection, an abnormal cylinder is identified from the central air-fuelratio when the rear oxygen sensor output is converged to a fixed valuewithin the predetermined range, and the change amount of the centralair-fuel ratio before diagnostics.

Lastly, description will be made on the case in which an air-fuel ratiovariation among the multiple cylinders in which one cylinder is leanerthan other cylinders has occurred. In FIG. 15( a), the air-fuel ratio ofthe No. 1 cylinder is shifted toward the lean side by an amount of Xle,and the air-fuel ratios of other cylinders are shifted toward the richside by an amount of Xle/(n−1). An optimum purification air-fuel ratiois determined by the air-fuel ratio of a rich cylinder. Al is the targetair-fuel ratio before diagnostics, Bl is the result of making a leanabnormality cylinder run rich, and Br is the result of making remainingnormal cylinders run rich. As shown in FIG. 15( b) as well, in a leanabnormality cylinder, the change amount of the central air-fuel ratio isless than an expected value Y0 (lean), and is larger than Y0 in normalcylinders (rich). In this case as well, the No. 1 cylinder whichexhibits a different tendency from others, showing a small change amountin the central air-fuel ratio is determined to be in a lean abnormality.

FIG. 16 shows a time chart when the No. 1 cylinder is in a leanabnormality. The air-fuel ratio of the No. 1 cylinder is controlled tobecome richer by an amount of X, and also the target air-fuel ratio andthe central air-fuel ratio are controlled to become richer by an amountof Y0. In this case, since as shown in FIG. 15( a), the optimumpurification air-fuel ratio is leaner, CO and HC will increase andthereby the output of the rear oxygen sensor in the downstream of thecatalyst will become rich. For this reason, the target air-fuel ratioand the central air-fuel ratio are corrected toward the lean side.

Implementing the above described embodiments, the following advantageswill be achieved. It is possible to detect an abnormal cylinder in anair-fuel ratio variation among the multiple cylinders from the centralair-fuel ratio when a predetermined cylinder is controlled to becomerich and a cylinder air-fuel ratio variation is intentionally generated.It is noted that abnormality of the upstream air-fuel ratio sensor willresult in a nearly same change amount of the central air-fuel ratiosince the degree of the air-fuel ratio variation is the same regardlessof in which cylinder the injection amount is increased. Thus, by usingthe change amount of the central air-fuel ratio, it is possible todetect only the air-fuel ratio variation among the multiple cylindersaccording to the present embodiment.

Moreover, since the detection error of the upstream air-fuel ratiodetection means due to air-fuel ratio variation among the multiplecylinders is compensated, exhaust emission performance will not bedeteriorated even if the air-fuel ratio variation among the multiplecylinders is intentionally varied. Further, during an abnormality whenthe air-fuel ratio among the multiple cylinders has varied, not only anabnormality can be detected, but also an abnormal cylinder can beidentified. As a result of this, the robustness of emission performanceas well as the maintainability during failure can be improved.

1. A control apparatus of an internal combustion engine, comprising: acatalyst for purifying exhaust emission discharged from multiplecylinders; upstream air-fuel ratio detection means for detecting anair-fuel ratio of exhaust gas that flows into the catalyst; and air-fuelratio control means for controlling a fuel injection amount of themultiple cylinders based on the upstream air-fuel ratio, wherein thecontrol apparatus is adapted to control the fuel injection amount of themultiple cylinders when the air-fuel ratio among the multiple cylindersvaries, such that the upstream air-fuel ratio is richer than theupstream air-fuel ratio detected before the air-fuel ratio among themultiple cylinders varies.
 2. The control apparatus of an internalcombustion engine according to claim 1, wherein a degree of control tomake the upstream air-fuel ratio richer is increased in accordance witha degree of the air-fuel ratio variation among the multiple cylinders.3. The control apparatus of an internal combustion engine according toclaim 1, wherein the internal combustion engine comprises downstreamair-fuel ratio detection means for detecting an air-fuel ratio of anexhaust gas that flows out from the catalyst, and the control apparatusdetermines an abnormality of the air-fuel ratio variation among themultiple cylinders based on the downstream air-fuel ratio detected afterthe air-fuel ratio among the multiple cylinders has varied.
 4. Thecontrol apparatus of an internal combustion engine according to claim 1,wherein the control apparatus comprises means for increasing theair-fuel ratio variation among the multiple cylinders by increasing ordecreasing a fuel injection amount of at least one cylinder by apredetermined proportion relative to fuel injection amounts of othercylinders.
 5. The control apparatus of an internal combustion engineaccording to claim 4, wherein a determination of air-fuel variationabnormality among the multiple cylinders is made if the downstreamair-fuel ratio detected after the air-fuel ratio among the multiplecylinders is varied departs from a predetermined range.
 6. The controlapparatus of an internal combustion engine according to claim 1, thecontrol apparatus being configured to estimate an oxygen storage amountto be stored in the catalyst from an accumulated value of differencesbetween at least the upstream air-fuel ratio and a central air-fuelratio at which three exhaust components consisting of HC, CO, and NOxreact with oxygen in exhaust gas without excess and deficiency, andcontrol the upstream air-fuel ratio such that the oxygen storage amountcomes into a predetermined range, wherein the central air-fuel ratio iscorrected toward the rich side in accordance with the air-fuel ratiovariation among the multiple cylinders.
 7. The control apparatus of aninternal combustion engine according to claim 6, wherein the controlapparatus is configured to increase the air-fuel ratio variation amongthe multiple cylinders by increasing or decreasing a fuel pulse widthonly for one particular cylinder; to correct the central air-fuel ratiotoward the rich side when the downstream air-fuel ratio becomes richerthan a predetermined range; to correct the central air-fuel ratio towardthe lean side when the downstream air-fuel ratio becomes leaner than apredetermined range; and to determine an abnormality of the air-fuelratio variation among the multiple cylinders based on a correctionamount of the central air-fuel ratio, the central air-fuel ratio beingcalculated while the air-fuel ratio variation among the cylinders isincreasing.